A hot runner system is used to produce plastic parts from a mold; the hot runner delivers molten material to the mold cavity through a manifold, a nozzle, and a gate. A gate vestige mark is created on plastic parts from a hot runner system; this mark is created at the interface between the nozzle, gate and the mold cavity. In an application that requires a good esthetic appearance the size and shape of the gate vestige is important. There are many variables which can worsen the appearance of the gate vestige mark on a part, including stringing. Stringing occurs when the melt at the front of the nozzle tip and in the gate area is still relatively molten and therefore fluid or flowable when the part is ejected. The melt or molten plastic is drawn out in a line or string as the part is ejected. Stringing necessitates cleanup of the injection nozzle and tip causing downtime of the injection molding apparatus, which can be quite substantial for some applications. If the melt is sufficiently frozen at the front of the nozzle when the part is ejected, the plastic breaks or shears off, leaving behind a clean gate vestige mark on the finished plastic part.
Although the problems of stringing and gate vestige are common to many injection molding applications, these problems are a particular concern when using molding resins such as polypropylene, and when using faster cycle times. In such applications, the nozzle is generally operated at a higher temperature or does not allow for significant cooling times.
Thus, there is a need for an injection molding nozzle and tip which reduces or eliminates the problems associated with stringing and gate vestige.